Document use of unsafe in compute kernels

[alamb]

Note: migrated from original JIRA: https://issues.apache.org/jira/browse/ARROW-10973

There are ~7 uses of unsafe in the compute kernels and we should document them according to the guidelines in the Arrow crate README

 
{code:java}
// JUSTIFICATION
// Benefit
// Describe the benefit of using unsafe. E.g.
// "30% performance degradation if the safe counterpart is used, see bench X."
// Soundness
// Describe why the code remains sound (according to the definition of rust's unsafe code guidelines). E.g.
// "We bounded check these values at initialization and the array is immutable."
let ... = unsafe { ... }; {code}

[ByteBaker]

There are no more unsafe blocks in the file. We can close this. @alamb

[alamb]

Thanks @ByteBaker